Balanced/unbalanced transformers (baluns) are a key component of double-balanced mixer and push-pull amplifier designs in wireless systems. They provide balanced outputs from an unbalanced input. Balanced output for wireless applications requires half the input signal amplitude at each of two output terminals, 180.degree. out of phase with each other. In principle, conventional transformer designs can produce this result. However, conventional transformer designs, typically wire wound devices, have an upper frequency limit of several hundred megahertz due to magnetic flux leakage and capacitive coupling between the windings. Current wireless applications require very high frequency operation at low power. Active balun designs provide high frequency but operate with high DC power consumption. Passive baluns are therefore preferred. Of the known passive balun designs, Marchand type devices have become the device of choice for wireless applications. They provide excellent balance and can be made in small, easily integrated, geometries. The preferred Marchand balun, from the standpoint of miniaturization, is the spiral coil type. A version of the spiral Marchand balun has been reported by T. Gokdemir et al., IEEE MTT-S Int'l Microwave Symp. Dig., pp. 401-404. They implemented the spiral balun using GaAs MMIC technology and two side-by-side spiral microstrip lines.
Chen et al. have also reported monolithic passive balun designs using meandered line configurations. See Chen et al., "Broadband Monolithic Passive Baluns and Monolithic Double-Balanced Mixer", IEEE Trans. Microwave Theory Tech., Vol. 39, No. 12, pp. 1980-1991. They use "rectangular spiral" configurations with air bridges to access the strip lines. A GaAs technology is used but the device geometry, in particular the strip line geometry, is not optimized for efficient integration.
A more compact balun design has been reported by M. Engles et al. See "A novel compact balun structure for multi-layer MMICs," Proc. 26.sup.th European Microwave Conf., pp. 692-696, September 1996. This design uses a stacked spiral configuration with one coil overlying another.
There continues to be a need for high frequency, low power, baluns that are inexpensive, and can be easily and efficiently integrated.